Past research demonstrates that alcohol and tobacco use frequently co-occur. The increased health risks associated with drinking and smoking king alone are substantial, and the comorbid use of the two substances produces a synergistic effect (e.g., multiplicatively increased rates of esophageal, laryngeal, and oral cancers). The literature supports a number of explanations for alcohol-tobacco comorbidity, including unidirectional (e.g., alcohol use causes tobacco use), reciprocal (alcohol use and tobacco use influence each other) and third variable (e.g., common etiology) explanations. However, extant research is limited by small samples with limited generalizability and cross-sectional designs that preclude testing of these competing theories. The current project will attempt to resolve the mechanisms that underlie alcohol-tobacco comorbidity using nationally representative samples, including the National Longitudinal Study of Adolescent Health, the Adolescent Health Risk Study, the Epidemiological Catchment Area Study, Monitoring the Future panel data, the National Longitudinal Survey of Youth, and the National Survey of Personal Health Practices and Consequences. A variety of multivariate longitudinal techniques, including autoregressive panel models, state-trait models, latent growth models, and latent profile analysis will be used to typify patterns of alcohol and tobacco use over time. These findings will be examined across gender, racial group, and age group using multi-group models. Further, the role of demographic (e.g., sex, race, age, SES) and psychosocial (e.g., emotional distress, behavioral undercontrol, life stress) third variables that predict concurrent alcohol and tobacco use will be investigated. This systematic prospective analysis of large representative samples will provide the most thorough examination of alcohol-tobacco comorbidity to date and should help to resolve the mechanisms underlying alcohol-tobacco comorbidity.